1. Field of the Invention
The present invention relates to the testing of a laser-initiated ordnance system and more particularly to a dual-wavelength low-power built-in-test technique for confirming optical continuity between a laser firing unit (LFU) and a laser-initiated device (LID) connected together by one or more fiber optic cable assemblies (FOCAs) in series. The technique uses a dual-wavelength spectrographic method that takes advantage of the spectral signature of a component of the LID.
2. State of the Art
Laser initiation systems are well known in the art, in which a light pulse is passed along a fiber optic cable and caused to impinge on and heat to ignition a pyrotechnic material. Laser initiation systems are safer than electrical initiation systems in that the former are not susceptible to inadvertent initiation by stray electromagnetic radiation. In addition to avoiding accidental operation, however, ordnance systems are also required to reliably operate upon occurrence of a predetermined stimulus. Continuity of a firing channel must therefore be confirmed. In the case of an electrical firing unit, verification of electrical continuity is relatively straightforward, although appropriate safety precautions must be observed. In the case of a laser firing system, verifying continuity becomes more difficult. The fiber optic cable may be severed, crushed or otherwise damaged.
One known method of testing optical continuity is optical time domain reflectometry (OTDR). An optical time domain reflectometer measures light scattered back toward the input as light moves down a fiber. OTDR creates a trace representing the reflection of light as a function of position along the fiber. Advantages of OTDR are that it shows where light is reflected along the fiber and requires access to only one end of the fiber. Significant disadvantages of OTDR are its high cost, bulky and heavy package, relatively short measurement range, need for a trained user, and lack of sufficient spatial resolution.
U.S. Pat. No. 4,917,014 describes a continuity test system employing in an initiator a dichroic filter which reflects light within one wavelength range for continuity test purposes and transmits light within a second wavelength range for ignition purposes. A wavelength selector in the laser firing unit selects between a test stimulus and an ignition stimulus. Light energy at the test wavelength is reflected by the dichroic filter back through the optical fiber. Thus, in a test mode of operation, the continuity test system indicates integrity of the optical system as a function of such reflected energy.
U.S. Pat. No. 4,862,802 describes various methods of verifying LID ignition by sensing pyro-generated light from the LID which propagates back up the optical cable to the laser. The same verification facility is used to determine if optical fibers and pyrotechnic elements have been properly connected, using the initiating laser source to emit a low-power test stimulus. Light reflected at the interface of the optical fiber and the pyrotechnic element experiences a change in intensity and phase from that of an unterminated fiber. Changes in intensity and phase may be detected to enable a user to verify proper connection. The described feature appears to relate to verifying the quality of a connection as the connection is made, as opposed to an automated post-connect or on-demand built-in-test. The built-in-test facility of the present invention, on the other hand, is fully automatic and not dependent on human intervention.
In large systems, a fiber optic conduit may include several fiber optic cable assemblies connected by in-line connectors. For example, in a rocket system, separate stages may each have their own fiber optic cable assemblies, these cable assemblies being joined by in-line connectors to form a single continuous fiber optic conduit. In such an instance, light reflected from the fiber interface surfaces of the in-line connectors is liable to have a similar magnitude of combined reflected energy as that of the light reflected from the LID interface; therefore, it can "swamp" the light reflected from the LID, preventing reliable detection. The present invention addresses this shortcoming.
A further shortcoming of prior art continuity test methods is the inability to explicitly test continuity to the surface of the pyrotechnic material itself, relying rather on reflectance from intervening optical elements of the LID. The present invention permits continuity testing to the surface of a pyro-technic material having spectrally-dependent reflectance.